Electronic communications, especially wireless communications, occur under varying channel conditions. In order to maintain a voice or data connection under unknown and changing channel conditions, many wireless communication system transmitters apply a margin. This margin reserves room for fading and other common adverse channel conditions during communication by ensuring that the quality of communications is not degraded below an accepted level if the channel is adversely affected by certain temporary severe channel attenuations. Thus, margin provides some insurance under adverse channel conditions.
A higher margin results in a higher level of robustness during changing channel conditions, but at the cost of a lower achievable data rate. Conversely, a lower margin results in a higher achievable data rate, but at the cost of increased vulnerability under changing channel conditions.
In many wireless communication system environments, headroom is used to provide margin. Headroom is the difference between the maximum power of the transmitter and the transmission power level required for a particular data rate. When a mobile station would like to establish a communication channel and has data in its buffer, the mobile station determines its maximum transmission power, subtracts the headroom, and uses the remaining available transmit power to determine a maximum data rate. Thus, the maximum data rate has margin built in to provide some protection against varying channel conditions. To date, headroom has been a constant value established to account for many (but not all) common adverse variances in channel conditions. One common model of adversely varying channel conditions is the Rayleigh fading model.
Although there are occasions where the channel varies more than the Rayleigh fading model predicts, there are many times when channel conditions do not vary as severely as Rayleigh fading. Under more benign channel conditions, such as Rician fading channel conditions, the standard headroom may be excessive. In other words, a transmission through more benign channel conditions would not exceed the maximum bit error rate even if the headroom were lower.